Add a generci hook class to output data using the FieldsIO helpers + demo + documentation.

[tlunet]

Following discussion in #533. Here is a first TODO list :

• Description of the base native data IO structure in the documentation
• Hook class to use the FieldsID
  • base serial implementation
  • MPI setup
  • demo and documentation
• Add an option for multiprocessing in Rectilinear.toVTR, which default to one proc

[tlunet]

But then, one critical question : if we make the FieldsIO classes the base IO feature of pySDC, shouldn't we make them compatible with python 3.9 and 3.10 ?